Make vertex paint affect mesh texture greyscale
A transparent shader converts the brightness of the input image to transparency. There is no such thing as "grey scale of a transparent shader", because the transparent shader only absorbs light (based on the color and value of it's input). To get a white feedback of the shader (independently of the background), you need to use a shader, which reflects the light based on the brightness of your attribute. To archive your objective, you have to input the factor of your attribute (your vertex paint layer) into the diffuse shader: